Fuel ethanol is acknowledged as a valuable alternative to fossil fuels. Economically viable ethanol production from the hemicellulose fraction of plant biomass requires the simultaneous fermentative conversion of both pentoses and hexoses at comparable rates and with high yields. Yeasts, in particular Saccharomyces spp., are the most appropriate candidates for this process since they can grow and ferment fast on hexoses, both aerobically and anaerobically. Furthermore they are much more resistant to the toxic environment of lignocellulose hydrolysates than (genetically modified) bacteria.
EP 1 499 708 describes a process for making S. cerevisiae strains able to produce ethanol from L-arabinose. These strains were modified by introducing the araA (L-arabinose isomerase) gene from Bacillus subtilis, the araB (L-ribulokinase) and araD (L-ribulose-5-P4-epimerase) genes from Escherichia coli. Furthermore, these strains were either carrying additional mutations in their genome or overexpressing a TAL1 (transaldolase) gene. However, these strains have several drawbacks. They ferment arabinose in oxygen limited conditions. In addition, they have a low ethanol production rate of 0.05 g·g−1·h−1 (Becker and Boles, 2003). Furthermore, these strains are not able to use L-arabinose under anaerobic conditions. Finally, these S. cerevisiae strains have a wild type background, therefore they can not be used to co-ferment several C5 sugars.
WO 03/062430 and WO 06/009434 disclose yeast strains able to convert xylose into ethanol. These yeast strains are able to directly isomerise xylose into xylulose.
Still, there is a need for alternative strains for producing ethanol, which perform better and are more robust and resistant to relatively harsh production conditions.